A few GURPS 3e combat rules questions

I'm a novice GM planning to run a GURPS 3e 17th-century buccaneer-themed session a few weeks from now. Some fine points I haven't figured out:

1. Fencing skill
You can parry twice with the Fencing skill - can you use both parries against the same attack if the first parry fails, or can the second parry only be used against a different attack?

2. Speed/range table
Whn figuring Speed for the speed/range table, do you always use the defender's raw Speed (i.e. (HT+DX)/2), or his Move score ({HT+DX)/2 rounded down), or just the number of hexes the defender moved last turn? The third option seems to be implied, but I'm not sure.

3. Retreat
Retreating gives you a +3 bonus to your Defense roll. Do you have to declare you are Retreating before you roll your Defense, or can you roll and then choose to Retreat only if the +3 bonus actually helps you? The first option seems preferable, but my experience is limited.

1. Different attack. Only way you can defend twice against the same attack is all out defending and using 2 DIFFERENT defenses (i.e. parry and dodge if parry fails)

2. Hexes moved that turn. But this is really only important if they are moving perpendicular to the ranged attacker. Moving towards or away does NOT give penalties except in terms of the new distance penalty

3. You must decide to retreat BEFORE you roll. This requires an available hex behind the defender only. I myself feel that retreating is kind of vague though (why would you NOT retreat unless there is a cliff behind you). I myself add an additional house rule that you can only retreat as part of a step maneuver. So if you step and attacked, you can't retreat that round. Or if you retreated already, you can't step and attack that same round (assuming your initiative is lower than your opponent). This is ONLY a house rule however!